1. Field of the Invention
The present invention relates to a process for growing compound semiconductors wherein Al.sub.x Ga.sub.1-x N films are grown on sapphire substrates, and particularly to an improvement in the process for growing compound semiconductors by means of organometallic vapor phase epitaxy (OMVPE).
2. Related Art Statement
Although Al.sub.x Ga.sub.1-x N has been expected as one of the most promising materials for blue or ultraviolet light emitting devices because of its direct band gap of 3.39.about.6.2 eV, its homogeneous crystals of high quality having large surface areas have so far been difficult to be provided by conventional processes for growing single crystals.
In conventional processes, GaN films are generally grown hetero-epitaxially on sapphire substrates by hydride vapor phase epitaxial processes (HVPE) using Ga--HCl--NH.sub.3 --N.sub.2 system. A GaN light emitting diode has an MIS type structure as shown in FIG. 7. Namely, it consists of: a sapphire substrate 1; a GaN layer grown on the substrate, consisting of n-type GaN film 2 and high resistive (insulating) GaN (i-GaN) film 3 doped with a considerable amount of Zn; and electrodes 5, 6 formed on the GaN layer. In this instance, it is necessary to control with a high precision the thickness of i-GaN film 3 within 1 .mu.m in order to determine the operating voltage of the light emitting diode. However, a high growth rate of no less than 30-60 .mu.m/hr. that is a suitable condition for obtaining high quality crystals in the aforementioned hydride vapor phase expitaxial processes, does not allow the thickness of the formed film 3 to be controlled precisely within 1 .mu.m, so that the operating voltage of the diode varies widely. Furthermore, for providing a high quality GaN layer, a thickness of at least 30 .mu.m grown on the sapphire substrate is generally required. Namely, even the n-GaN film shown in FIG. 7 has to be 30 .mu.m thick or more. However, if GaN grows beyond a certain extent of thickness on the sapphire substrate, cracks 12 are formed in the grown GaN layer and the sapphire substrate as shown in FIG. 7, owing to differences in lattice constant and thermal expansion coefficient between the grown GaN and the sapphire. The cracks in the wafer, particularly formed right below the electrode, may cause a current leakage and thus lead to the disadvantage of low production yields in the manufacture of light emitting diodes.
Therefore, organometallic vapor phase epitaxial processes with an organometallic compound system, such as trimethylgallium (TMG)--NH.sub.3 --H.sub.2 system have been investigated, aiming at precise thickness controls of i-GaN films and growth of thin GaN films 10 .mu.m thick or less, with a decreased growth rate. In such processes, an apparatus for deposition as shown in FIG. 2 is used. Namely, in a quartz tube reactor 7, a sapphire substrate 10 is placed on a graphite susceptor 9 for high-frequency induction heating, the sapphire substrate is heated by means of radio-frequency coil 8, then gaseous TMG, NH.sub.3 and H.sub.2 introduced from a raw material supply tube 11 are impinged on the substrate 10 and thus GaN films grow on the substrate 10. As procedures to be followed in this case, firstly, the substrate 10 is cleaned by heating at about 1,100.degree. C. under H.sub.2 flow, after which the temperature of the substrate is lowered to about 970.degree. C. and then gaseous TMG and NH.sub.3 are fed to grow GaN.
As the result, the growth rate is lowered to 1-3 .mu.m/hr., whereby growth of uniform thin films 1-4 .mu.m thick can be achieved. This is an advantage having never been obtainable by hydride vapor phase epitaxial processes. However, the thin films have rough surfaces where hexagonal columnar grains are observed, and their reflection high energy electron diffraction patterns (RHEED pattern) show a spot pattern characteristic of a single crystal having finely rugged surfaces, whereas, on the other hand, GaN films formed in accordance with hydride vapor phase epitaxial processes exhibit a streak line pattern characteristic of smooth surfaces.